This invention relates to a microencapsulation system, to a method for preparing the microencapsulation system and to pressure-sensitive transfer sheets employed in this system.
At the present time, microencapsulation systems including pressure-sensitive transfer sheets utilizing microcapsules containing an oily liquid and a colorless dye intermediate are well known in the art. In these systems, the sheet is formed of a suitable substrate such as paper having coated thereon microcapsules comprising a polymeric wall which surrounds an oil droplet containing the colorless dye intermediate. Generally, the microcapsules are coated on the substrate by utilizing a polymeric binder composition. In use, the coated surface of the transfer sheet is positioned against an underlying copy sheet and pressure is supplied to the uncoated surface of the transfer sheet to rupture the capsules and effect transfer of the dye intermediate to the underlying copy sheet. The copy sheet contains a composition which is reactive with the dye intermediate to form visible colored marks at that portion of the surface of the copy sheet adjacent to the capsules which have been ruptured and from which the dye intermediate has been transferred.
While this transfer system has proven to be satisfactory in applications wherein it is desired merely to form an image on the copy sheet corresponding to the image formed under pressure on the uncoated surface of the transfer sheet, less than satisfactory results have been obtained in other applications. For example, in a transfer system wherein it is desired not only to form the copy on the copy sheet but it is also desirable to permit the user to write on the coated surface of the transfer sheet with commonly employed oil-based inks such as are employed in ball-point pens, the written image obtained is often incomplete. This is because the pressure needed to effect the writing is sufficient to rupture the microcapsules thereby releasing oil which admixes with the applied ink and contaminates the printing press or the ball-point pen tip thereby interrupting the ink flow. Thus, attempts to write on the coated surface with a ball-point pen typically result in clogging and skipping, similar to the effect observed when trying to write on a greasy or oily surface.
Prior attempts to solve these problems including changing the nature of the polymeric composition used to form the microcapsules or applying a protective coating over the microcapsules have proven to be unsatisfactory in that they cause a material reduction in the transfer of oil and dye intermediate from ruptured capsules to the copy sheet under normally employed writing pressure. Accordingly, it would be desirable to provide a means which would permit printing or writing with oil-based inks on the coated surface of the transfer sheet without adverse effects on the transfer of the oil-based dye intermediate from ruptured microcapsules.